1. Field of the Invention
The present invention relates to an afocal lens, more specifically, relates to an afocal lens for an optical head that optically records and reproduces information onto different types of information recording media such as an optical disk, etc., the optical head having the afocal lens, an optical disk apparatus provided with the optical head, and optical information equipment provided with the optical disk apparatus.
2. Description of the Related Art
In recent years, as blue-violet semiconductor lasers have come into practical use, a Blu-ray Disk (hereinafter referred to as BD) having the same size as a CD (Compact Disk) or a DVD (Digital Versatile Disk) and being an optical information recording medium (hereinafter referred to as an optical disk) of high density and high capacity has also been put into practical application. The BD is an optical disk with a protective substrate being about 0.1 mm thickness that performs recording or reproduction by using a blue-violet laser source having wavelength of about 400 nm and an objective lens with the numerical aperture (NA) increased to 0.85.
On the other hand, a HD DVD with a protective substrate being about 0.6 mm thickness that also uses a blue-violet laser source having wavelength of about 400 nm and an objective lens with the numerical aperture of 0.65 is also in practical use.
Hence, there is proposed a compatible optical head that uses one objective lens to focus laser beam onto information recording surfaces of optical disks having different thickness of the respective protective substrates, thereby recording and reproducing information.
Each of Japanese unexamined patent publication No. 7-98431 and Japanese unexamined patent publication No. 10-10308 describes an optical head provided with a light-condensing optics system that is capable of focusing laser beam to the diffraction limit, onto optical disks having the protective substrates with different thicknesses.
FIG. 18 shows an example of configuration of a conventional optical head described in the Japanese unexamined patent publication No. 7-98431. In FIG. 18, an optical head 20 includes a light source emitting red laser beam 1, a beam-splitter 3, a collimating lens 4, a hologram lens 5, an objective lens 6, a detection lens 8, and a light receiving element 9. In addition, reference symbol 70 designates the DVD that is an optical disk with a protective substrate being 0.6 mm thickness.
An operation of the optical head 20 that records and reproduces information onto/from the DVD 70 is described. A red laser beam emitted from the light source 1 transmits through the beam splitter 3, is transformed into substantially parallel light by the collimating lens 4, transmits through the hologram lens 5, and is focused as a light spot on an information recording surface of the DVD 70 through the protective substrate. A returning red laser beam that is reflected at the information recording surface of the DVD 70 transmits through the objective lens 6, the hologram lens 5, and the collimating lens 4 on the same light path as the advancing path. Then, the returning red laser beam is reflected by the beam splitter 3, given predetermined astigmatism by the detection lens 8, guided by the light-receiving element 9. The red laser beam then generates an information signal and a servo signal.
Next, with reference to FIG. 19, an operation of the optical head 20 is described in the case where recording or reproduction is performed on a CD 80 that is an optical disk having the protective substrate being 1.2 mm thickness. The red laser light emitted from the light source 1 transmits through the beam splitter 3, and is transformed into substantially parallel light by the collimating lens 4. Then, after being diffracted by the hologram lens 5, the red laser beam is focused as a light spot on an information recording surface of the CD 80 by the objective lens 6 through the protective substrate. A returning red laser beam that is reflected at the information recording surface of the CD 80 transmits through the objective lens 6, the hologram 5, and the collimating lens 4 on the same light path as the advancing path. Then, the returning red laser beam is reflected by the beam splitter 3, given predetermined astigmatism by the detection lens 8, guided by the light-receiving element 9. The returning red laser beam then generates an information signal and a servo signal.
Generation of a focus error signal for recording or reproducing the DVD 70 and CD 80 is capable of using a so-called astigmatism method, etc. that detects a light-condensing spot to which the detection lens 8 gives astigmatism, by means of a four-divided light-receiving pattern in the light-receiving element 9. In addition, a tracking error signal is capable of using a so-called 3-beam method or differential push-pull method (DPP method), etc., that uses main beam and sub-beams generated by a diffraction grating (not shown).
In the following, a function of the hologram lens 5 and the objective lens 6 will be described with reference to FIG. 20, FIG. 21A and FIG. 21B.
As light is focused as a minute light spot to the DVD 70 and CD 80, the hologram lens 5 is provided with a grating pattern 5a as shown in FIG. 20. A diffraction efficiency of a plus-first-order diffracted light of the hologram lens 5 is less than 100%. The hologram lens 5 is designed so that transmitted light (hereinafter in this application, transmitted light that is not diffracted may also be represented as a zero-order diffracted light, and thus the transmitted light is treated as one diffracted light) has also sufficient intensity. In addition, the hologram lens 5 can increase the sum of light quantity of the zero-order diffracted light and the plus-first-order diffracted light through blazing thereby improving the light utilization efficiency.
The objective lens 6 has the numerical aperture (NA) of 0.6, and, as shown in FIG. 21A, is designed to form a light-condensing spot of the diffraction limit on the DVD 70 with the protective substrate being 0.6 mm thickness, when laser beam that transmits through the hologram lens 5 without being diffracted (in other words, the zero-order diffracted light) enters.
On the one hand, as shown in FIG. 21B, the plus-first-order diffracted light that was diffracted at the hologram lens 5 is focused on the CD 80 by the objective lens 6. Herein, the plus-first-order diffracted light is subjected to aberration correction so as to form a light-condensing spot of the diffraction limit on the CD 80 with the protective substrate being 1.2 mm thickness.
Thus, a combination of the hologram lens 5 that diffracts part of incident light with the objective lens 6 can enable implementation of a bifocal lens capable of forming a light-condensing spot that will be focused to the diffraction limit on each of optical disks having different substrate thickness.
In addition, since the hologram lens 5 has the lens action, positions of two focal points in a light axis direction differ. Thus, while information is being recorded or reproduced using a light spot formed on one focal point, the other light spot formed on the other focal point widely expands, and does not affect recording or reproduction of information.
Use of such the optical head 20 can enable recording or reproduction of information onto optical disks of different kinds, respectively, by using one objective lens.